Telephone system



April 8 1941., A. A. LUNDsTRoM TELEPHONE SYSTEM Filed Nov. 28, 1939 13Sheets-Sheet 1 ATTORNEY A. A. LUNDSTROM TELEPHONE SYSTEM April 8, 1941.

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TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 10 NMS /NVE/VTOAALU/VDSTROM TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 11April 8, 1941. A. A. LUNDSTRM- TELEPHONE SYSTEM Filed Nov. 28, 1939 13Sheets-Sheet 12 MON .00nd \Iy my. `Q NON hmm /NVENTOR By A.,4./.u/vasmouAT7'ORNEV April 8, 1941 A. A. LUNDsTRoM 2,237,742

TELEPHONE SYSTEM Filed Nov. 28, 1939 l5 Sheets-Sheet 15 /Nl/EN To@ A. A.LUNDSTROM y l ATTORNEY Patented Apr. 8, 1941 TELEPHONE SYSTEM Alexis A.Lunastmm, am ons, N. s., saigner to ,Bell Telephone laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationNovember 28, 1939, Serial No. 306,466

25 Claims.

invention relates to telephone systems and `more particularly to one inwhich the designation oi a wanted subscriber is transmitted by currentsof different frequencies generated at the calling station by a deviceoperatively responsive to the manipulation of a key-set. In certainautomatic telephone systems the subscriber station is provided with arotary dial, and when the establishment of a connection is desired, thisdial is operated a number of times to produce trains oi impulses whichare transmitted to the exchange over the line wires and there receivedeither on a succession of step-by-step selectors which are directvelyoperated in response to the impulses to reach the called subscribersline or upon so-called senders which function to directlvely set theselectors under the control of its own pulses, produced in response tothose produced at the calling station but recorded in the sender.

Both of these methods of establishing auto matic connections, whilesatisfactory from an operating standpoint, are, nevertheless,susceptible ol distinct improvements, on the one hand from the callingsubscribers viewpoint who, in dialing as many as nine different trainsof impulses for certain types of connections, may, in waiting for thedial to return home during the production oi each train of impulses,forget how many digits have already been dialled and, therefore, make itnecessary for him to start the establishment oi the connection all overagain thus incurring loss of time; and on the other hand also from theoperating companys viewpoint which, in dial-operated systems and becausethe quantity ci central office equipment is, among other things, afunction of the time taken to dial a number, must provide more equipmentthan would otherwise be necessary if the time taken to transmit a wanteddesignation could be appreclably reduced below that required by the useoi the dial.

According to my invention a calling subscriber communicates the ofilceand number cfa wanted subscriber by means which control the productionof alternating or pulsating currents of various irequencies, said meanspreferably compris lng a set of depressible keys any one of which, whendepressed, causes alternating currents of particular frequency orfrequencies to be produced for transmission over the line wires. Thesealternating current may be generated at the callers instrument as, forexample, by means of a number of reeds which, in response to thedepression of a key, are set into vibration within a magnetic field inthe combination required to produce the frequencies corresponding to thekey depressed.

According to one feature of my invention the subscribers station isarranged, with respect to the receiving equipment at the central cniceto which the line extends, to avoid the production of currents otherthan the signal frequencies produced by the current generatinginstrument at the subscribers station. In operating with signalfrequency transmission it has been found desirable to use alternatingcurrents having frequencies lying within the voice range, and in theembodiment of the present invention in which the current generatingdevice is located at the subscriber's station and is equipped with reedsthat have natural periods within lthis range, the equipment at thecentral oflice which responds to the currents produced by these reedscannot distinguish between these currents and. say, other currentswithin the voice range which may be produced by sounds that impinge uponthe telephone transmitter during the act of operating the caller'ssending instrument. To avoid the production of these false currents itis necessary to so dispose the reed generator across the line during thetransmission of the signals that practically no line current passesthrough the transmitter during the generation of the signal frequencies.According to this feature of my invention a rectifier network isprovided for the transmitter and the reed generator whereby, onsignaling, the network presents a high loss to any line current thatwould flow to or from the transmitter but little or no impedance to theflow of the alternating line current through the coil of the instrumentthat produces the signal frequencies, On the other hand, when theconnection has been set up, the direction of the line cur 4e rent isreversed so lthat, in this case, the rectifier network presents littleor no impedance to the flow of direct or alternating line currentthrough the transmitter and a high impedance to the flow of alternatingcurrent through the coil of the sending instrument.

According to another feature of my invention a sender is provided at thecentral office in which the alternating currents produced by the reedgenerator and transmitted over the line wires control the operation ofas many channel detectors as there are signal frequencies, each detectorbeing responsive substantially to a single frequency, to cause theoperation of a relay. The combination of two relays operated for asingle digit is then employedto record on settable regis- 'I'he settingof the registers is'then utilized in the known manner to cause theoperation of switches for selectively reaching the wanted line.

Yet according to another feature of my invention 4the intensity of thedial tone transmitted to the calling station is controlled by thedistance of the station from the home oi'iice in order that theintensity of the tone to the several stations at diil'erent distancesfrom the omce shall be uniform.

According to another feature of my invention a delay circuit is providedwhereby the channel detectors are not made responsive to the incomingsignals for a short interval subsequent to their arrival, in order thata portion of the energy delivered to them may be dissipated before thedetectors are made available for responsive operations. The reason forthis is to make sure that relays which will be operated by the 2oactivation of the individual detectors will not be so by a falseresponse of any detector to the transient disturbance set up in all thechannel filters by any sudden arrival of the signals at their inputs dueto the depression of a key at the calling instrument. In this way therelays will not be subject to false operations from transients ofdurations shorter than a predetermined interval.

Yet according to another feature of my invention this delay circuit isadapted to paralyze or otherwise render inactive any of the detectorchannels if the input signal is of greater intensity than apredetermined value, and, also, to keep`the detectors paralyzed untilthe disturbance has passed below the Just operate level of the delaycircuits for a predetermined interval of time. If the disturbance thengoes above this z Fig. 4 is the outgoing part of the line-nder switchincluding the sender connector:

Fig. 5 shows the private branch exchange selector PBX, a final connectorC and wanted station B; while Figs. 6 to 13, inclusive disclose thesender.

Considering nrst Fig. 1, it will be seen that the subscribers station Ais equipped with a reed plucking current generating device III. A typ-1d] ical instrumentality of this kind is that shown level it must, ofcourse, persist for longer than another predetermined interval to renderthe detector channel eiIective.

Yet another feature of my invention is the use of a special tunedfeedback arrangement in the detector channel which renders the operationof the relay controlledv thereby more sensitive to the incomingfrequency and renders the release of the relay more quickly thanotherwise when the signal current is removed. This makes for positiverelay action which expedites and renders reliable those functions of thesender that depend upon the operation and release of the channel relays.

These and other features of my invention Iwill be readily ascertainedfrom the following detailed description of the invention, appendedclaims and attached drawings in which:

Fig. 1 is a schematic representation of the invention as embodied in aconnection between the calling subscriber's station A and the calledsubscribers station B over a train of switches selectively positioned bya sender responsive to the signaling frequencies generated at thecalling station;

Fig. 1A shows the arrangement of Figs. 2 to 13, inclusive to disclosethe complete invention:

Fig. 2 shows a subscribers station circuit equipped with a reed currentgenerating device and rectler network for controlling current throughthe station transmitter and effectively switching from talking to reedgenerator circuit and vice-versa for conversation or signalingrespectively;

Fig. 3 is a combined line circuit and the line connecting side of aline-finder switch of a private branch exchange;

in Patent 2,147,710, issued to R. F. Mllina on February 21, 1939.Briefly, this instrument comprises flve reeds, tuned, respectively, toas many diiferent frequencies and mounted to modulate the field of apermanent magnet which, in turn, causes a voltage to be induced in acommon pick-up winding connect to the line. Any two of the reeds areplucked simultaneously by the depression of one of the ten key buttonsand are thus caused to vibrate at their natural period to producealternating currents determined by their diiferent and respectiveperiodicities. It is understood, however, that the alternating currentgenerating device located at the subscribers premises is not limited tothe one above-mentioned and briefly described but may be any suitableinstrument for producing 8181181 frequencies either singly or combinedwith means responsive to the operation of a key 0r similar device.

In order to disclose the invention in its broadest aspects and thusbring out its nexibility with respect to its adaptation to telephonesystems employing diiferent types of equipment and used for establishingdifferent kinds of connections, said invention is applied by way ofillustration to the establishment of telephone connections over a systeminvolving a calling sulmcriber's line located in a private branchexchange which is provided with mechanical switching facilities that areselectively operated te connect with trunks radiating in the severaldirections through which desired connections may be established inaccordance with the number transmitted from the calling station. Theswitching structures herein used for illustrating the operation of theinvention are of the well known step-by-step or Strowger type andreference may be had to pages 53 to 67, inclusive. of the second editionof Automatic Telephony by Smith and Campbell for a detailed descriptionof the operation of the circuits associated with these switches. Onlythose portions of the circuits of the selector and connector switchesare shown as are required for a clear and complete description of thisinvention, the omitted portions of these circuits being indicated bybroken lines. It should be understood, however, and the description ofthe operation of the invention will amply show, that the invention mayJust as readily be applied to any other type of switching structure as,for instance, the so-called panel or cross-bar type, and themodification of the controlling circuits for adapting the invention tothese or any other type of switches may be easily made by anyone skilledin the art.

Referring now more particularly to Fig. 1. there are shown a callingsubscribers station A equipped, as said before, with a reed currentgenerating device 20|, the associated line finder LF which includes theline circuit w, omitted from Fig. 1 but shown in detail in Fig. 3, a.sender connector SC, a private branch exchange selector switch PBX,intermediate selector switches, indicated but not shown, a connectorswitch C and a called subscriber-s station B. Directly belovi the senderconnector are a plurality of senders designated by the first sender andlast sender. In brief, the invention functions as follows:

When the subscriber A lifts the receiver from the cradle, the linefinder LF is operated to move its brushes to the terminals of the line,after which an idle sender is connected to the line finder by theoperation of a relay 405 of whatever sender happens to be free, thesenders being disposed in a preferred order for distributive connectionsamong the several groups of line finders serving the different groups oflines. The linefinder switch LF', however, is one connecting element ofa switch structure which comprises the line-finder switch proper havingaccess to calling line terminals, the sender connector element 405 bywhich an idle sender is connected to the calling line via the brushes ofthe lineilnder switch and a selector switch structure that has access,through its brushes, to the different trunl; groups leading to theseveral points of destination.

New when the sender becomes connected to the line, the private branchexchange switch becomes operatively associated with the sender and ittransmits dial tone" thereto just as it would transmit it to the callingsubscriber to notify him that he may proceed to dial if it wereconnected directly to the subscribers line. In my invention, however,the switch PBX, instead of being selectively positioned by impulsesproduced at the calling station as it would be in a dial operatedsystem, will be selectively positioned by impulses produced by thesender, and since the sender can produce these switch-stepping impulsesonly in response to the call designation as keyed by the subscriber andrecorded in the sender, dial tone cannot be transmitted from theselector directly to the calling station, but from the -sender to thestation after the sender has become operatively connected to theselector. The tone which is transmitted by the selector PBX to thesender operates equipment therein which produces another tone that isthen transmitted over the line to the calling station.

In response to this tone, the subscriber operates device to transmit thewanted designation by depressing a keyfor each letter or digitrepresenting the wanted number, and the depression of each of the keyswill cause two of the reeds to vibrate at a definite frequency, eachthereby producing a current of that frequency. 'I'hese currents aretransmitted over the line into the sender which is provided with afrequency detector channel for each signal frequency. The particularchannels tuned to the transmitted frequencies are operated, after asmall time iterval to allow for the dissipation of transients, and causethe operation of a relay in each voi! the responding channels. Obviouslythe-"successive depression of different keys will vproduce a successivecombination of different frequencies which will cause the operation of adifferent combination of detector channels? and relays operated inresponse thereto. if.'

In combination with the five detector channels there is afiiliated aspecial delay system network. This system has the function vofconnecting the channel filters to the input side of the line after apartof the signal energy has dissipated itself and, some time later, tosensitize the channel detector units. The purpose of this is, as saidbefore, to avoid in the signal frequency the effect of components ofother frequencies present which, unless kept out of a particular signalband, will tend to decrease the frequency discriminating qualities ofthe channel filters by setting up -an additional frequency spectrum. Theadditional delay thereafter, before the channel detectors aresensitized, is necessary in order to allow the spectrum resulting fromsuddenly applying the signals to the filters at the end of the firstdelay to subside. This permits the filters to realize a good portion oftheir steady state frequency discriminating characteristics and thusenable the detectors to respond to larger variations in loss due to loopdifferences and other causes without obtaining false operations inadjoining channels.

The delay circuit serves three other functions in addition to thosementioned. The first is to block any signal of less than a predeterminedduration; the second is to block any signal which has an intensity of acertain predetermined value above the just operate level of the channelsand the third is to block the operation of the channels until the signalpasses below the just operate level of the delay network to apredetermined lower level. If the signal then goes above the :lustoperate level after dropping below the predetermined lower level, itmust persist for a period longer than the interval of time controllingthe second function mentioned above before the channels will beactivated.

The combination of relays operated in response to the incomingcombination of two frequencies that corresponds to the depression of akey is recorded in a. group of register relays which are locked topreserve the record, and every time the channel relays operate inresponse to succeeding combinations of frequencies, a correspondingrecord is locked in another and succeeding register, each digit of thecalled line designation thus being recorded on a separate register,there being provided in the sender as many registers as the maximumnumber of digits that may be required to set up a connection to any partof the telephone area.

Once the called number is recorded on the sev-v eral registers, thesender proceeds to use the registrations successively to produce thenumber of impulse trains required to directively position all theselectors necessary to reach the wanted line. According to Fig. 1 thesubscriber will have to key one digit for setting the private branchexchange selector PBX, and the number of levels to which the brushes ofthis selector will be caused to step by the sender will depend upon thedirection of the call; lthe destination from the private branch exchangeoutlet coni trolling, in any case, the number of additional digitsrequired. If, for instance, the call is to terminate before the manualboard of another Aprivate branch exchange, one digit will suffice todirectively set the private branch exchange switch PBX on the bank levelcontaining the trunks that lead to this board. On the other hand, if theconnection is for a called subscriber B, reached through an automaticcentral oflice. a maximum of nine digits maybe required, that is, onedigit for directively setting the Aprivate branch exchange switch PBX tothe outlet level for reaching the office selectors, three digits for thedesignation of the wanted office in order that the oflice selectors maybe positioned for connection, to a trunk extending to the wanted oiiice,four digits for the directive setting of the selectors in the oiiice forreaching the terminals of the wanted line and an additional digit for apossible party line or ten thousand digit designation i'or the wantedline. When the appropriate number of selectors has been positioned bythe corresponding number oi pulse trains produced by the sender, thesender provides a suitable signal, in the form of a circuit foroperating relay 408, for closing the connection through, after which thecalled line B is rung in the known manner and the sender is disconnectedfrom the line by the release of relay 405, in readiness to set upanother call.

Referring now more particularly to Figs. 2 to 13, the completeunderstanding of the invention may be obtained from a consideration ofthe circuit operations which follow the initiation of a call and thetransmission from the subscribers station of the wanted subscribersnumber.

Assuming that line A is located in the second level of terminals in thelower bank 320 of the two banks of terminals 320 and 330 accessible tothe brushes of line nder LF and that a call is initiated from said lineby the removal of the handset 200 from its cradle. As a result,ott-nornormal switchhook contacts 203, 204 and 203 are closed and acircuit is completed for the subscribers line relay 302 which extendsfrom battery through the right winding of said relay, No. l. contacts ofcut-01T relay 303, line conductor 2I0, swtchhook contacts 204, one armof the rectifier bridge consisting of rectiers 201 and 206, primarywinding ofthe induction coil 202, swtchhook contacts 209, line conductor2`II, No. 3 contacts of cut-off relay 303, left winding of line relay302 to battery. Relay 302 operates and through its No. 2 contacts closesa circuit for operating group relay 306 while through its No. 1 contactsand conductor 304 it connects the winding of cut-off relay 303 to thesleeve terminal of the line in bank 340. Relay 306 operates anddisconnects ground from conductor 301 so as to mark the secondcommutator segment of all of the line-nders having access to the groupof lines containing line A, and connects ground to start conductor 308.Assuming the line finder LF to be the rst idle nder in the group, theground on conductor 308 is extended through the normal No. 1 contacts ofrelay 3I3 to the winding o relay 3I0. Relay 3l@ operates, therebyclosing a circuit for energizing relays 3II and 3I2, the circuit for thelatter relay being traced from battery through its winding, through thebrush and normal segment of commutator 3I4, to ground at the No. 2contacts oi' relay 3I0 and the circuit for the former being traced frombattery through resistance 329, No. 4

contacts of relay 3 I 9, through the normally closed contacts of rotarymagnet 3Ill, winding of relay 3l I, back contact of the vertical magnet3I3, and through the No. 2 back contacts of relay 3I2, to ground at theNo. 2 contacts of relay 3I0. When relay 3I2 operates, the circuit foroperating relay 3H may be traced through the No 2 front contact of relay3I2 (instead of through the No. 2 back contact) and thence through thebrush and normal segment of commutator 3M, to ground at the left frontcontact of relay 3I0. Relay 3H is Vlocked operated through its frontcontacts to ground on the No. 2 contact of relay 3m, independent ofrelay 3I2 and independent of the commutator 3I4, while relay 3I2 islocked operated through its No. 2 front contact and the front contactsof relay 3I I to the ground at the No. 2 contacts of relay 3II).

With both relays 3II and 3I2 operated, a circuit is closed for operatingthe vertical magnet 75 3 I 3, said circuit being traced from batterythrough its winding, the No. 1 front contacts of relay 3I2, the outerset of contacts of relay 3I I, to ground at the No. 2 contacts of relay3I0. The operation of vertical magnet 3I3 raises the shaft and allmultiple and commutator brushes up to the iirst level of line terminalsand opens the circuit through the winding of relay 3II in consequence ofwhich said relay releases and opens the circuit through the Winding ofmagnet 3 I 3, which releases. Now if the terminals of the calling linewere located in the first level of the terrnlnal bank, relay 3 I 2 wouldrelease when its locking circuit is opened by the release of relay 3l Isince its own operating circuit, opened when the brush of commutator 3I4was advanced from its normal position, would not be closed on the firstsegment of commutator due to the disconnection of ground from thissegment by the operation of relay 306. It has been assumed, however,that the terminals of the calling line are not located in the firstlevel. Therefore relay 3 I2 will be held operated through the brush andiirst segment of commutator 3I4, to ground at the back contact of thegroup relay associated with the lines which do connect to terminals inthe first level. Relay 3II is then reoperated through the No. 2 contactsof relay 3I2, to ground on the iirst off-normal segment of commutator3I4. When relay 3II reoperates, the circuit for energizing the verticalmagnet 3I3 is closed once more and the reoperatlon of the magnetaccordingly steps the shaft and brushes up to the second level. Withmagnet 3 I 3 operated, however, relay 3II again releases, whereupon thecircuit through the Winding of the magnet is again opened and relay 3I2is unlocked. Since the ground normally connected to the second segmentof commutator 3|@ has been disconnected therefrom by the operation ofrelay 306, relay 3I2 releases to prevent the further operation of magnet3I3. The calling line being connected to bank 320 to which brushes 32|and 322 have access, it is not necessary to operate the brush switchingrelay 324; and since relay 306 was operated through its left-handwinding, no ground is connected to conductor 309 to cause the operationof relay 325 over the circuit that would be completed through its rightcontacts and the brush accessible to the segments of commutator 3I5reserved `for the lines located in the upper terminal bank 330.

As soon as the shaft was moved upward from its normal position, thevertical off-normal springs 3I6 were operated to connect ground toconductor 325l and to prepare for the operation of the rotary magnet3Il. The release of relay 3I2, after the brushes have been advanced tothe calling level, again closes a circuit to ground on the No. 2contacts for operating relay 3l l, and the reoperation of this relaycloses a circuit for operating the rotary magnet 3H, said circuitextending from battery through the winding of the magnet, the No. e andNo. 5 contacts of the vertical oi-normal springs 3I6, the No. 1 backcontact of relay 352, the front contacts of relay 3H to ground at theNo. 2 contacts of relay 3M. Magnet 3H operates and advances the lowerbrushes 32E and 322 to the first set of terminals in the selected level,opens the circuit for operating relay 3H and connects the winding ofthis relay through the front contacts of magnet 3I7, the lower windingof relay 3I8, and the No. 1 normal contacts of relay 324 to the testbrush 323. if the calling une is connected to the erst set of terminalsin the selected level, the circuit through the winding of relay 3II isextended over brush 323, over the sleeve conductor 304 of the callingline, through the No. 1 contacts of the operated 1ine relay 302, throughthe winding of the cut-off relay 303 to battery, thereby holding relay3I| operated to prevent the further advance of the brushes. If thecalling line is connected to some other set of terminals in the selectedlevel, then relay 3|| is not held operated and its release will causethe release of magnet 3H. When this magnet releases, relay 3IIreoperates, causing thereby the reoperation of magnet 3|1. Relay 3|I andmagnet 3|1 are, ln this manner, alternately operated and released untilthe test brush 323 engages the sleeve terminal to which conductor 304 isconnected, at which time relay 3|I is held operated in series with thewinding of cut-off relay 303 over the path previously traced. Thecut-off relay 303 operates and relay 3|3 is sufficiently energized bythe current through its lower winding to close a locking circuit frombattery through its upper winding and No. 2 front contacts, through thefront contacts of magnet 3H, the winding of relay 3l I, the back contactof magnet 3|3, and through the front contacts of relay 3| I to ground atthe No. 2 contacts of relay 3|0. Relay 3|8, with the aid ofy the currentilowing through its locking winding is now completely operated to extenda connection from line A through brushes 32| and 322, the No. 2 and No.4 back contacts of relay 324, and the No. 1 and No. 4 contacts of relay3|3, over conductors 35| and 352, No. 3 and No. 4 back contacts of relay408, both windings of relay 40 I both windings of relay 403, the upperwindings and the lower windings oi both relays being serially connected,respectively, with the upper winding of relay 403 bridged by rectifier442 and lower winding bridged by rectifier 443, resistance 401 tobattery, and resistance 409 to ground. Relays 40| and 403 operate toperform functions which will be shortly described except to note at thistime that relay 40| closes an obvious circuit for relay 402 which, inoperating, applies ground over its No. 3 contacts to the holdingconductor 353 which functions to hold relay 3|0 in an operated conditionover a locking circuit through the No. 3 contacts of relay 3|9, No. 2contacts of relay 3|8, upper winding of said relay to battery and,further, to hold operated the brush transfer relay 324, if operated,over a locking circuit extending from ground on said conductor, No. 2and 3 contacts of vertical olf-normal springs 3|6, No. 3 contacts oi'relay 324, winding of said relay to battery. It further closes a circuitfor relay 4|3 over the contacts of relay 4|2 which operates but performsno function at this time. Relay 3|9 operates in a circuit through itsleft winding, the No. 3 front contacts of relay 3|8 and the No. 1 and 2contacts of vertical off-normal springs 3|6, locks to ground onconductor 306 should there be another calling line in the same level ineither bank to keep the group relay 306 operated from the contacts ofthe line relay of said other line, extends this vrovnd over its No. 1contacts to conductor 328 which is the start conductor in the nextline-finder switch of the group that will be taken into use and, at itsNo. 4 contacts, disconnects battery through resistance 329 from thewinding of relay 3| I, and opens the circuit of relay 3|0 which releasesand closes a circuit path for the left winding of relay 3I9 through itsNo. 1 back contacts to off-normal ground on conductor 321. The purposeof this circuit is, of course, to insure that if another call isinitiated in the group during the interval when the line finder LF isreleasing and, therefore, when its brushes have not been completelyreset, the releasing line finder will not be re.. seized by thereclosure of the circuit for relay 3|0 and the consequent operation ofvertical magnet 3| 3; for, in such an event. the brushes may be arrestedfrom returning to normal and restarted in their upward movementfrom'some position other than normal. Therefore by holding relay 3|!!operated to ground on the vertical oir-normal contacts 3|6 until thebrushes reach normal, any call initiated prior to this time Will havethe start ground on conductor 308 extended. to conductor 328 and thenceto relay 3I0 oi.' whatever line-finder switch in the group is normal andselectable.

When relay 40| operates, it closes an obvious circuit for relay 402which operates as already described. Relay 402 is made slow-release sothat any momentary accidental depression of the switchhook which maycause the release of relay 40|, said last mentioned relay being heldover the line loop will not, at the same time, cause the release ofrelay 402 and thereby disturb the circuits established through itscontacts. Relay 402, on operating, connects battery through its No. 1contacts to the Winding of a relay in the group of relays 404 to 404 ofthe first idle sender available to the group of line finders servingline A, characterized by the normal position of the sender busy relay406 to 406".

Each line finder, depending upon the trafc density of the send-er group,has a number of sender connectors as an integral'part of its circuitorganisation, each sender connector comprising a relay 405, sender busyrelay 406 and a sender selecting relay 404, each connector givingaccessibility to one sender. The designations of the relays justmentioned refer to the first connector in Fig. 4 which -is also nrst inthe order of selection. while all intermediate connectorrs havesimilarly designated relays that are primed (those of the last connectorbeing double primed) to indi-cate identity of function in the differentconnectors.

Now when a sender is made busy through operative association with alineilnder in any group or through the removal of the sender from service, acircuit is closed for the upper Winding of relay 406 in every senderconnector through which the busy sender may be avail-able. Postponi-ngfor the moment the tracing of the current path involved for this relay,it will be observed that if relay 406 operates as a result of the senderconnect-able to the rst sender connector being made busy, said relaywill lock in a path extending from battery through its lower winding andlower front contacts, lower back cont-acts of all intermediate senderbusy relays 406' to 406" which are normal because their associatedsenders are free, conductor 420 to ground on the No. 2 contacts of relay402. Similar locking circuits will, of course, be avail-able to theother sender -busy relays in the event that the associated senders arenot free.

If the senders available to the sender connectors forming an integralpart of the line finder LF are all free, then, when ground is connectedto the No. l contacts of relay 402, the sender accessible through therst sender connector will be selected for operative association withsaid line finder and this is accomplished by operating relay 404 over acircuit path which extends from battery on the No. 1 contacts of relayl402,

conductor 429, upper back contacts of relay 406, winding of relay 404,lower back contacts of said relay, through the back contacts of otherrelays 404', intermediate .the first sender-connector and the last, toground on relay 404" of 'the last sender-connector. On the other hand,if all senders available to these connectors are busy except theintermediate sender and, therefore, all rel'ays 406 to 406" are operatedand locked except the intermediate relay 406', then the senderaccessible through the intermediate sender-connector will be selected bythe operation of relay 404' over a circuit which extends from battery onconductor 429, upper front contacts of relay 406, upper fron-t contactsof relays 406' of rall senderconnectors between the rst and theintermediate connector, upper back contacts of relay 406', winding ofrelay 404', bottom back contacts of relays 404', bottom back contacts ofrelays 404 of all sender-connectors between the intermediate connectorsand the last connector to ground on the bottom back contact of relay404" In the same manner if all senders are busy except the oneaccessible through .the last sender-connector, the above circuit will betraced through the top front contacts of all relays 406 to 406', topback cont-acts of the last relay 406", winding of relay 404" to groundon the bottom back contacts of said relay. Thus it is seen that a chainpreference lock-out circuit is provided whereby the line nder LF iscaused to be connected to the idle sender available through the firstsenderconnector which is not busy, a condition which is denoted by thenon-operated condition of the sender busy relay 408 of the senderavailable' through that channel.

For the purpose of continuing with the description of the operation ofthe system, however, it may be assumed that the sender connecte-ble tothe line finder LF through the first senderconnect-or is idle and that,therefore, as a result, relay 404 operates in the manner abovedescribed,

Upon the oper-ation of relay 404 a circuit is completed for relay 405which extends from ground on the top back contact of relay 404",serially through the top back contacts of all other relays 404' in eachof the connectors, which relays are non-operated by the opening of theirrespective circuit paths through the top contacts of their correspondingsender-busy relays 406 as soon as these relays operate in response tothe non-availability of their respective senders, top front cont-acts ofrelay 404, Winding of relay 405 to battery. Relay 405 operates toperform functions noted hereinafter.

With relay 405 operated, ground is connected over its No. l contacts toconductor 4|4 and thereby completes a circuit for relay |300 whichoperates to perform a number of functions to be noted hereinaf-ter.Relay 405 over its No. 2 contacts connects the ring conductor 422 toconductor 4|5 (via conductor 422') which, in turn,

joins conductor 4|6 through the No. 4 contactsof relay |200 whenoperated; over its No. 3 oont-acts relay 405 extends the tip conductor423 to conductor 4|6 (via conductor 423'); over its No. I

contacts it extends the other line conductor 352 over conductor 4|9 tothe other side of said resistance bridge to form the fourth arm thereof;over its No. 7 contacts it connects the winding of relay 408 toconductor 420 and thence to conductor via the No.{2 contacts of relay938 and conductor, thence to the No. 2 contacts of relay |309 which,when operated, cause relay 408 to operate as described hereinafter; overits No. 8 contact-s it opens the operating circuit of its associ-atedsender busy relay 406 which, however, will not release at this timesince it is locked over its lower winding and contacts to ground on theNo. 2 contacts of relay 402 as already described; while over its No. 9contacts it connects battery through resistance 401 to conductor 430which extend-s to the grid of vacuum tube 6|2.

In the present embodiment of the invention, lt is assumed lthat theconnection is to be completed to subscribers line B located in anautomatic office equipped with step-by-step switches of the Strowgertype, the A or first digit office selector of which can be reached fromthe terminal banks of .the PBX selector shown at the left of Fig. 5 tothe windings of whose line relay 5|5 the tip and ring conductors 423 and422 are respectively extended. Now when relay 405 operates and these twoconductors are joined to conductors 4|6 and 4| 5, respectively, as abovedescribed, a current path is completed for relay 5| 5 which extends frombattery, lower winding of relay 5|5, No. 5 back contacts of relay 5|4,conductor 422, conductor 422', No. 2 contacts of relay 405, conductor4|5, primary winding of transformer 628, conductor 621, No. 2 normallymade contacts of relay I2', conductor 4|6, No. 3 contacts of relay 405,conductor 423', conductor 423, No. 1 back contacts of relay 5|4, upperwinding of relay 5|5, Nos. 1 and 2 contacts of 10th Rot. step rotaryolf-normal springs 524, secondary winding of transformer 522 to ground.Relay 5|5 operates and closes an obvious circuit for relay 5|6 whichalso operates and, in doing so, applies ground to hold conductor 5|3.

Inasmuch as the primary winding of transformer 522 is connected to asource of dial tone, said tone is induced over the operating circuit ofrelay 5|5, above traced, including the primary winding of transformer628. This tone, therefore, is further induced into the secondary windingof said transformer, resulting in the operation of detector-amplifier6|3. This detectoramplifer may be of any suitable type wherein an energysupply is activated to operate relay 600 upon the induction of the tonecurrent in the secondary winding of transformer 628, and the operationof relay 600 closes an obvious circuit for relay 6|4 which operates andconnects generator 603 to the network B20. The current produced by thegenerator 603 is transmitted through the network 620, and through thefilter 6|0, but the quantity of energy that gets through will dependupon thyrite resistance elements 601 and 608 bridged across the network,the shunting effect of which is controlled by the quantity of currentflowing through the anode circuit of vacuum tube 6|2 of which saidelements form a part.

It will be remembered that the operation of the sender-connector relay405 extendedthe line conductors 35| and 352 to conductors 4|8 and 4|9,respectively, and that these, in turn, are connected to the fourth armof the resistance bridge SI5. Now the two vertical terminals of thisbridge are connected to the primary winding of transformer 62| while itstwo horizontal points are connected to the primary winding oftransformer 6 I 9.

Referring again to relay 403 which, it will be recalled, operated overthe line loop, the flow of current through resistance 401 causes apctential drop measured by this resistance, and the current flowingtherethrough, that is applied to conductor 430 which, as said before, isconnected to the grid or control element of vacuum tube 6l2. This tubeis of the heater" type having a thyrite resistance element 606 in serieswith its cathode. Prior to the application of the potential drop throughresistance 401 to the grid of the tube, lthere is a slight currentflowing through its anode circuit which extends from battery, thyrite606, the cathode (the filament of the tube being heated through a seriesbattery connection with the filament of vacuum tube SII), anode of tubeSI2, the two thyrite elements 601 and 600 to ground. Thyrite is selectedto supply the fixed negative bias on tube SI2 because it has theelectrical characteristic that potential drop across itself increasesvery slowly with increase in current through itself. Therefore,variation in anode current of tube 6I'2 will not materially affect thenegative potential drop across thyrite 606 and it will remain almostfixed. If there is no potential drop or a very slight potential drop inresistance 401 due to the factthat, for instance,

the line loop is long, the grid bias is strongly negative because of theapplication to the cathode of the negative potential drop through the-thyrite element 606. The current flowing through the anode circuit is,under these circumstances, very small. Conversely, ifthe potential dropfrom resistance 401 applied to the grid is large as would be the case,for instance, when the loop is small, the resulting grid bias due to thesum of the positive potential drop across resistance 401 and thenegative potential drop across thyrite B06 would be less negative. Inconsequence, the anode current would be large. In other words, thevoltage drop across thyrite element 606 works ln opposition to thevoltage applied to the grid from the loop so that, on long loops, thetotal grid bias will be highly negative to permit very little current toflow through the anode circuit of the tube while on a short loops thetotal negative grid bias will be small to permit a very large current toflow through the anode circuit. Consequently y the potential dropapplied to the grid of tube E l2 and the quantity of current which thenflows through its `anode circuit, above traced, depends upon the lengthof the loop, including the resistance of conductors 2I0 and 2H thatextend l between the subscribers station A and the terminals on theline-finder bank 320 at the telephone office. Whence it follows that thecurrent flowing through the anode circuit of vacuum tube BIZ, beingdependent upon the value of the positive potential on the gridof thetube, is a maximum for zero line loop and a minimum for the longest lineloop. The anode current, however,`

circulates through the thyrite elements 601 and 608, which have theproperty of decreasing in resistance with an increase in current flowingtherethrough. For the small current through the anode circuit producedby a long line loop, the current through the thyrite elements is slight,

the resistance thereof remains comparatively high and the shuntingeffect of these elements, bridged across the network 620 is very small.Hence for a long line loop, by which is meant the ohmic distance of thecalling subscribers station A from the oflice in which the lineterminates,

most of the energy from the generator 603 will be transmitted directlyinto the filter 6|0, without any appreciable quantity of it passingthrough the shunting thyrite bridge. Onthe other hand, when the loop issmall or of negligible resistance, the current through the anode circuitis large, the thyrite elemerts decrease their resistance in response -tothe par sage of the current and the effect is to produce a shunt pathacross the network 620 -that will cause most of the energy from thegenerator 603 to flow through the shunt circuit with little of itpassing into the filter 6 I0.

The reason for the above arrangement is to regulate the quantity of dialor keying tone current transmitted back to the calling station inaccordance with the length of the loop so that the intensity of the tonewill be practically uniform for all stations regardless of theirdistance from the oiiice. In dial telephone systems in which thedesignation of the wanted line is transmitted by means of direct currentimpulses, no such case need be exercised about the tone intensity at thestation because the tone used is usually of low frequency, preferablywithin the voice range, say around 500 to 600 cycles. 'Since the lineloss increases with the frequency used, the loss in the intensity of thetone at this frequency (500 to 600 cycles per second) is small fordifferent loo-ps and the same energy level, therefore, may be applied toall lines regardless of their distance from the oice. But in the presenttelephone system in which the designation of the wanted line istransmitted by means of combination of frequencies within the voicerange, the frequency of the dial or keying tone used cannot be withinthe same range as the signal frequencies because the keying tonetransmission network is coupled to the channel detectors responsive tothese signal frequencies and such detectors therefore, may be falselyoperated in response to the dial current. To avoid any such falseoperations, the keying tone frequency is chosen well above the rangeused for signaling from the calling station. Since the line loss isgreater with the higher frequency than it is with the frequency used indial systems. it is evident that if the tone level ls suitable for themost remote station it will be too loud for the nearest station whichfact makes it necessary to make the quantity of tone transmitted overthe line depend upon the length of the loop as above described. Thuswhen the line is long the quantity of keying tone current transmittedthereover will have to be sufficient to provide for ohmic andattenuation losses over it whereas, when the line. is short and theelectrical constants `making for such losses are not present, thequantity of tone current is reduced in proportion to allow for theproduction of the same intensity of sound at the station receiver.

The keying tone is produced by the generator 603 which generates the oddharmonics of some base frequency say, for example, 20 cycles, and thepass-band filter 6I0 has an upper and lcwer frequency cut-ofi whichdefines the harmonics to be produced by the generator 603. Thus if thelter is designed to pass all frequencies between an upper limit of 1480cycles and a lower limit of 1400 cycles. the generator should be adaptedto produce the '11st or 73rd harmonic of the 20 cycles and either orboth of these harmonies would be transmitted through the iilter andtransformer SIS to the horizontal terminals

